LeetCode-Binary Tree Level Order Traversal

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本文深入解析了二叉树层次遍历的概念、原理，并通过Java代码实例展示了如何实现层次遍历。文章从理解二叉树结构出发，逐步引入层次遍历的重要性，最终通过实际代码实现，帮助读者掌握层次遍历算法的应用。

题目：https://oj.leetcode.com/problems/binary-tree-level-order-traversal/

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree {3,9,20,#,#,15,7},

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

分析：使用两个队列来进行分层遍历。

源码：Java版本
算法分析：使用栈。时间复杂度O(n)，空间复杂度O(1)

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> results=new ArrayList<List<Integer>>();
        if(root==null) {
            return results;
        }
        Queue<TreeNode> currentQueue,nextQueue,tempQueue;
        currentQueue=new LinkedList<TreeNode>();
        nextQueue=new LinkedList<TreeNode>();
        currentQueue.add(root);
        TreeNode node;
        while(!currentQueue.isEmpty()) {
            List<Integer> list=new ArrayList<Integer>();
            while(!currentQueue.isEmpty()) {
                node=currentQueue.poll();
                list.add(node.val);
                if(node.left!=null) {
                    nextQueue.add(node.left);
                }
                if(node.right!=null) {
                    nextQueue.add(node.right);
                }
            }
            results.add(list);
            //swap currentQueue and nextQueue
            tempQueue=currentQueue; 
            currentQueue=nextQueue;
            nextQueue=tempQueue;
        }
        return results;
    }
}